A transformer has become an essential electronic component for voltage regulation into required voltages for various kinds of electric appliances. Referring to FIG. 1, a schematic exploded view of a conventional transformer is illustrated. The transformer 1 principally comprises a magnetic core assembly 11, a bobbin 12, a primary winding coil 13 and a secondary winding coil 14. The primary winding coil 13 and the secondary winding coil 14 are overlapped with each other and wounded around a winding section 121 of the bobbin 12. An isolating tape 15 is provided for isolation and insulation. The magnetic core assembly 11 includes a first magnetic part 111 and a second magnetic part 112. The middle portion 111a of the first magnetic part 111 and the middle portion 112a of the second magnetic part 112 are embedded into the channel 122 of the bobbin 12. The primary winding coil 13 and the secondary winding coil 14 interact with the magnetic core assembly 11 to achieve the purpose of voltage regulation.
Since the leakage inductance of the transformer has an influence on the electric conversion efficiency of a power converter, it is very important to control leakage inductance. Related technologies were developed to increase coupling coefficient and reduce leakage inductance of the transformer so as to reduce power loss upon voltage regulation. In the transformer of FIG. 1, the primary winding coil 13 and the secondary winding coil 14 are overlapped with each other and wounded around the bobbin 12. As a consequence, there is less magnetic flux leakage generated from the primary winding coil 13 and the secondary winding coil 14. Under this circumstance, since the coupling coefficient is increased, the leakage inductance of the transformer is reduced and the power loss upon voltage regulation is reduced, the electric conversion efficiency of a power converter is enhanced.
In the new-generation electric products (e.g. LCD televisions), a backlight module is a crucial component for driving the light source because the LCD panel fails to illuminate by itself. Generally, the backlight module comprises a plurality of discharge lamps and a power supply system for driving these lamps. The discharge lamps are for example cold cathode fluorescent lamps (CCFLs). These discharge lamps are driven by an inverter circuit of the power supply system. As the size of the LCD panel is gradually increased, the length and the number of the lamps included in the LCD panel are increased and thus a higher driving voltage is required. As a consequence, the transformer of the inverter circuit is usually a high-voltage transformer with leakage inductance. For electrical safety, the primary winding coil and the secondary winding coil of such a transformer are separated by a partition element of the bobbin. Generally, the current generated from the power supply system will pass through a LC resonant circuit composed of an inductor L and a capacitor C, wherein the inductor L is inherent in the primary winding coil of the transformer. At the same time, the current with a near half-sine waveform will pass through a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switch. When the current is zero, the power MOSFET switch is conducted. After a half-sine wave is past and the current returns zero, the switch is shut off. As known, this soft switch of the resonant circuit may reduce damage possibility of the switch, minimize noise and enhance performance.
Referring to FIG. 2, a schematic exploded view of a transformer used in the conventional LCD panels is illustrated. The transformer 2 of FIG. 2 principally comprises a magnetic core assembly 21, a first bobbin piece 22, a second bobbin piece 23, a primary winding coil 24 and a secondary winding coil 25. The first bobbin piece 22 has a first side plate 26. The second bobbin piece 23 has a second side plate 27 and a plurality of partition plates 23a. Several winding sections 23b are defined by any two adjacent partition plates 23a. According to voltage dividing principle, the number of winding sections 23b may be varied depending on the voltage magnitude. In addition, a first base 26a and a second base 27a are extended from the first side plate 26 and the second side plate 27, respectively. Several pins 28 and 29 are respectively arranged on the bottom surfaces of the first base 26a and the second base 27a. 
For winding the primary winding coil 24 on the first bobbin piece 22, a first terminal of the primary winding coil 24 is firstly soldered on a pin 28a under the first base 26a. The primary winding coil 24 is then successively wound on the first bobbin piece 22 in the direction distant from the first side plate 26. Afterward, a second terminal of the primary winding coil 24 is returned to be soldered onto another pin 28b under the first base 26a. For winding the secondary winding coil 25 on the second bobbin piece 23, a first terminal of the secondary winding coil 25 is firstly soldered on a pin 29a under the second base 27a. The secondary winding coil 25 is then successively wound on the winding sections 23b of the second bobbin piece 23 in the direction distant from the second side plate 27. Afterward, a second terminal of the secondary winding coil 25 is returned to be soldered onto another pin 29b under the second base 27a. Moreover, due to the partition plate 23a of the second bobbin piece 23, the primary winding coil 24 is separated from the secondary winding coil 25, thereby maintaining an electrical safety distance and increasing leakage inductance of the transformer 2.
The winding structure of the transformer 2, however, still has some drawbacks. Since the transformer 2 is applied to the driver circuit of the power supply system, a higher driving voltage is required. If the voltage difference between the primary winding coil 24 and the secondary winding coil 25 is too high or the safety distance is insufficient, the transformer 2 is readily suffered from high-voltage spark. Moreover, since the magnetic core assembly 21 is partially exposed and disposed adjacent to the primary winding coil 24 and the secondary winding coil 25, the safety distance between the winding coils and the magnetic core assembly 21 is insufficient. In addition, since the primary winding coil 24 and the secondary winding coil 25 are returned back to be respectively soldered onto the pins 28b and 29b under the first base 26a and the second base 27a, portions of the primary winding coil 24 and the secondary winding coil 25 are exposed under the first bobbin piece 22 and the second bobbin piece 23. Even if the exposed portions are covered by insulating material, the safety distance is also insufficient. Under this circumstance, the transformer 2 is readily suffered from high-voltage spark or short circuit and eventually has a breakdown. For complying with the circuitry layouts of different power supply systems, the transformer manufacturers need to make a variety of bobbin molds. Under this circumstance, the fabricating cost is increased and the material management is very important.
Therefore, there is a need of providing a composite transformer so as to obviate the drawbacks encountered from the prior art.